Various methods for adjusting image density of an image forming apparatus are known in the related art. For example, a method in which light is reflected off a toner image and detected by an optical sensor so as to estimate the amount of toner adhesion and then a developing bias voltage is controlled based on the estimated toner adhesion amount to adjust the image density. Another method of adjusting the image density is by replenishing toner in a developing unit of the image forming apparatus or by discharging toner.